Compensator for polarization-mode dispersion compensation

Abstract

A compensator for compensating polarisation-mode dispersion, comprises a splitter ( 30 ) arranged to split light, according to the light's polarisation, between a first optical path and a second optical path; a combiner ( 40 ) arranged to combine light from the first optical path and the second optical path, and a monitor ( 15 ) for monitoring the light. An active wavelength-delay device ( 70 ) is provided in the first optical path and arranged such that components of the light travelling in the first optical path and having different wavelengths are subjected to different delays, the delays being selected according to an output from the monitor ( 15 ).

The invention claimed is: 
     
       1. A compensator for compensating polarisation-mode dispersion, comprising: a splitter arranged to split light, according to the light's polarisation, between a first optical path and a second optical path; a combiner arranged to combine light from the first optical path and the second optical path, and a monitor for monitoring the light; characterised in that the apparatus comprises an active wavelength-delay device arranged in the first optical path such that components of the light travelling in the first optical path and having different wavelengths are subjected to different delays, the delays being selected according to an output from the monitor, wherein the active wavelength-delay device comprises an active region of an arrayed-waveguide grating (AWG). 
     
     
       2. A compensator as claimed in  claim 1 , in which the active wavelength-delay device is arranged to compensate for chromatic dispersion due to second-order polarisation-mode dispersion. 
     
     
       3. A compensator as claimed in  claim 2 , in which the group delay is dependent upon an output from the monitor. 
     
     
       4. A compensator as claimed in  claim 1 , further comprising an active group-delay device arranged in the first or second optical path, wherein the active group-delay device provides a group delay to light passing along the first or second optical path relative to light passing along the other of the first and second optical paths. 
     
     
       5. A compensator as claimed in  claim 4 , which is arranged to compensate actively, using the group-delay device, for first-order polarisation-mode dispersion and group delays due to second-order polarisation-mode dispersion. 
     
     
       6. A compensator as claimed in  claim 1 , in which the active wavelength-delay device is arranged so that the components having the different wavelengths are subjected to delays that vary quadratically with wavelength, so that a linear frequency chirp is imparted to the light, the magnitude of the chirp being actively variable. 
     
     
       7. A compensator as claimed in  claim 1 , in which the AWG comprises a second active region that is the active group delay device. 
     
     
       8. A compensator as claimed in  claim 1 , in which the compensator further comprises a polarisation controller. 
     
     
       9. A compensator as claimed in  claim 1 , further comprising a further active wavelength-delay device arranged upstream or downstream of the splitter. 
     
     
       10. A compensator as claimed in  claim 9 , in which the further active wavelength delay device is arranged to provide wavelength-dependent delays selected according to the output from the monitor. 
     
     
       11. A compensator as claimed in  claim 1 , further comprising an active wavelength-delay device arranged in the second optical path, such that components of the light travelling in the second optical path and having different wavelengths are subjected to different delays. 
     
     
       12. A compensator as claimed in  claim 11 , in which the wavelength dependent delays are selected according to the output from the monitor. 
     
     
       13. A compensator as claimed in  claim 11 , in which the active-wavelength-delay devices arranged in the first and second optical paths subject components of light travelling in their respective paths to wavelength-dependent delays having opposite signs. 
     
     
       14. A compensator as claimed in  claim 1 , further comprising at least one pair of active wavelength-delay devices, one device of each pair being in the first optical path and one in the second optical path, the pair being arranged to compensate third- or higher-order chromatic dispersion due to polarisation mode dispersion. 
     
     
       15. A method of compensating polarisation-mode dispersion in a light signal, comprising inputting the light into a compensator, monitoring an indicator indicative of polarisation-mode dispersion on the light, directing a first polarised component of the light along a first optical path, directing a second, differently polarised, component of the light along a second optical path and recombining the polarised components from the first and the second optical paths, characterised in that the method includes the step of delaying different wavelengths of the light travelling in the first optical path by different amounts that are dependent upon the monitored indicator, wherein one or more active regions of an arrayed-waveguide grating (AWG) are adjusted to provide the wavelength-dependent delays. 
     
     
       16. A method as claimed in  claim 15  that includes the step of introducing a group-delay to the component passing along the first optical path relative to the component passing along the second optical path, the group delay being dependent upon the monitored output. 
     
     
       17. A method as claimed in  claim 15  that includes the step of adjusting an active region to provide the group delay of the component passing along the first optical path. 
     
     
       18. A method as claimed in  claim 15 , further comprising the step of delaying different wavelengths of the light travelling in the second optical path by different amounts that are dependent upon the monitored indicators. 
     
     
       19. A method as claimed in  claim 15 , in which the one or more active regions are adjusted to provide a delay for compensating second-order chromatic dispersion. 
     
     
       20. A method as claimed in  claim 19 , that includes the step of adjusting a further pair of active regions, one in the first optical path and one in the second optical path, to provide wavelength-dependent delays for compensating third-or higher-order dispersion. 
     
     
       21. A compensator for compensating polarisation-mode dispersion, comprising: a splitter arranged to split light, according to the light's polarisation, between a first optical path and a second optical path; a combiner arranged to combine light from the first optical path and the second optical path; an active group-delay device, arranged in the first optical path, for delaying the light in the first optical path relative to the light in the second optical path; a monitor for monitoring chromatic polarisation-mode dispersion on the light; and an active wavelength-delay device arranged upstream or downstream of the splitter, components of the light having different wavelengths being subjected to different delays in the active wavelength-delay device, the different delays comprising delays selected to compensate chromatic dispersion in light entering the compensator, characterised in that the different delays further comprise delays selected according to an output from the monitor, wherein the active wavelength-delay device comprises an active region of an arrayed-waveguide grating (AWG). 
     
     
       22. A compensator as claimed in  claim 21 , in which the monitor is arranged upstream of the splitter. 
     
     
       23. A compensator as claimed in  claim 22 , in which the monitor is arranged downstream of the combiner. 
     
     
       24. A compensator as claimed in  claim 21 , in which the active wavelength-delay device is arranged to provide compensation for a positively dispersed signal. 
     
     
       25. A compensator as claimed in  claim 21 , further comprising a wavelength-delay device, arranged in the first or the second optical path for providing compensation for a negatively dispersed signal. 
     
     
       26. A method of compensating polarisation-mode dispersion in a light signal, comprising: inputting the light into a compensator; splitting the light into a first polarised component and a second, differently polarised, polarised component; directing the first polarised component along a first optical path; directing the second component of the light along a second optical path; delaying the component passing along the first optical path relative to the component passing along the second optical path; recombining the polarised components from the first and the second optical paths; monitoring an indicator indicative of a chromatic-dispersion component of the polarisation-mode dispersion of the light; and delaying different wavelengths of the light, by different amounts, in order to compensate chromatic dispersion of the light that is input into the compensator, wherein one or more active regions of an arrayed-waveguide grating (AWG) are adjusted to provide the wavelength-dependent delays, characterised in that the method further comprises delaying different wavelengths of the light by different amounts in order to compensate the chromatic-dispersion component of the polarisation-mode dispersion of the light indicated by the monitored indicator.